Surface Enrichments

Fig. la —c. Schematic drawing of some specific examples of polymer molecules at an interface (a) the free surface of a homopolymer, (b) the surface enrichment of one component in a miscible polymer blend, and (c) the interface between polymers of different molecular weight and/or chemical composition... 

When dealing with polymer blends or blockcopolymers, surface enrichment or microstructures may be observed as already discussed in Sect. 3.1. Quite similar effects may be expected for buried interfaces e.g. between polymer and substrate where one component may be preferentially enriched. In a system of PS, PVP and diblock copolymer PS-6-PVP it has been shown by FRS that the copolymer enrichment is strongly concentration dependent [158]. In a mixed film of PS(D) and end-functionalized PS on a silicon wafer the end-functionalized chains will be attached to the silicon interface and can be detected by NR [159],... 

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

It has generally been assumed that the most important consideration in the surface enrichment of one metal in preference to another in a supported bimetallic cluster is based on differences in the enthalpies of sublimation of the metals which comprise the cluster. In most cases, the surface composition is enriched in the metal having the lower enthalpy of sublimation (1 ). 

Surface Composition Measurements. The surface composition and metal dispersion for a series of silica (Cab-O-Sll) supported Ru-Rh bimetallic clusters are summarized In Table I. Surface enrichment In Rh, the element with the lower heat of sublimation, was not observed over the entire bimetallic composition range. In fact, to within the experimental limit of error of the measurements, surface compositions and catalyst compositions were nearly equal. A small local maximum In the dispersion was observed for the catalyst having a surface composition of 50% Rh. 

As was the case for the silica-supported Ru-Rh bimetallic catalysts, there was no significant surface enrichment in either metal over the entire range of bimetallic catalyst compositions. 

The surface-catalyst composition data for the silica-supported Ru-Rh cuid Ru-Ir catalyst are shown in Figure 1. A similcir plot for the series of silica-supported Pt-Ru bimetallic catalysts taken from ref. P) is included for comparison purposes. Enthalpies of sublimation for Pt, Ru, Rh and Ir are 552, 627, 543, and 648 KJ/mole. Differences in enthalpies of sublimation (a<75 KJ/mole) between Pt and Ru cind between Rh and Ru are virtually identical, with Pt euid Rh having the lower enthalpies of sublimation. For this reason surface enrichment in Pt for the case of the Pt-Ru/Si02 bimetallic clusters cannot be attributed solely to the lower heat of sublimation of Pt. Other possibilities must also be considered. 

The surface properties of three types of methanation catalysts obtained by oxidation of selected Intermetallics were examined In relation to their CO conversion activity. The first type (Ni Si, N1 A1 ) which corresponds to active phase-supporl iX the coXventionally prepared catalyst Is little affected by the oxidation treatment. The surface Nl is oxidized and relatively more abundant In the active solids. The second type (active phase-promoter ex Ni Th ) is extensively decomposed on oxidation. The transformation of these alloys Is accompanied by a surface enrichment in Nl. 

N1 surface concentrations determined from ESCA are plotted as a function of bulk N1 content in Figures 1 and 2. In the case of homogeneous alloys the points should fall on the 45 diagonal line. It can be seen that In both (N1 SI ) and (N1 Th ) series the surfaces of the alloys are nickel-poor, Ss compared to tHe bulk. Similar observations have been made In the case of N1 A1 (11,12) and Co Th (13) alloys. Surface enrichment In Si or tS i2 to be expected be cause of the higher heats of formation of S10 and ThO, compared to NiO (-210, -292, and -58.4 kcal/mol, respectively). This would lead to a higher chemical affinity of SI and Th toward the ambient gas and consequently an Increased driving force of SI and Th for segregation. 

For ail samples, both a.p. and s.o., irrespective of the preparation method, the experimental intensity ratios, V2p/Zr3d, increased proportionally to the V-content up to 3 atoms nm 2 (pjg 2). The ratio approaches those calculated with the spherical model proposed recently by Cimino et al. [27] (full line in Fig. 2). For ZV samples with V-content < 3 atoms nm 2, this finding shows that vanadium species are uniformly spread on the Zr02 surface. On ZV catalysts with a larger V content (not shown in Fig. 2), the intensity ratios were markedly larger than the corresponding values yielded by the spherical model. The results obtained on samples with V-content > 3 atoms nm 2 point therefore to a V surface enrichment. 

Recently characterization of bimetallic nanoparticles by EXAFS were extensively reported [122-124,176], Structural transformation of bimetallic Pd/Pt nanoparticles, which were prepared by a sequential loading of H2PtClg onto the Pd loaded catalyst, was investigated with EXAFS at high temperatures [176], The results of EXAFS at Pd K and Pt L-III edges showed that Pt was surface-enriched or anchored on the Pd metal core with an increase of the Pt content. The structure of the obtained bimetallic Pd/Pt nanoparticles seemed to be retained upon heating up to 1273 K under ambient condition [176], Pt/ Au bimetallic nanoparticles can be prepared by polyol method and stabilized by PVP [122], XANES and EXAFS studies were also performed on the samples and their results supported the idea of a Pt-core/Au-shell structure with the elements segregated from each other [122],... 

It is clear that the measured surface enrichment profiles agree well with the theoretical predictions. 

This argument with respect to the ionic radii would predict a surface enrichment of Sn4+ (0.083 nm) for the ternary oxide, which is not observed. Hutchings et al. [50]... 

Powder XR diffraction spectra confirm that all materials are single phase solid solutions with a cubic fluorite structure. Even when 10 mol% of the cations is substituted with dopant the original structure is retained. We used Kim s formula (28) and the corresponding ion radii (29) to estimate the concentration of dopant in the cerium oxide lattice. The calculated lattice parameters show that less dopant is present in the bulk than expected. As no other phases are present in the spectrum, we expect dopant-enriched crystal surfaces, and possibly some interstitial dopant cations. However, this kind of surface enrichment cannot be determined by XR diffraction owing to the lower ordering at the surface. 

Beck, L., S. Bosonnet, S. Reveillon, D. Eliot, and F. Pilon (2004), Silver surface enrichment of silver-copper alloys A limitation for the analysis of ancient silver coins by surface techniques, Nuclear Instrum. Meth. (B) 226(1-2), 153-162. 

It has indeed been found (83a) that the work function of Ag-Pd alloy films equilibrated at 300°C only changes from 4.38 eV at pure silver to 4.50 eV at 86% Pd (increasing to 5.22 at pure Pd). It was proposed (83a) that this work function pattern is a consequence of surface enrichment by silver and so confirmation of the theory awaits further work function measurements on alloy surfaces for which compositions have been determined by, say, Auger electron spectroscopy. 

The use of hydrogen annealing to ensure homogenization and surface cleanliness is an attractive procedure in alloy film preparation but for the question of surface enrichment. The possibility of surface enrichment in... 

The GPR survey data showed a consistent zone of resistance located at 0.5-1 m below the surface for each of the five E-W transects. Stacking the graphical images of the transect data, according to their locations on the plan, reveals a tongue-shaped structure of resistance that extends southwards (down slope) at least 200 m visible on all five transects. The depth of resistance is consistent with a sub surface enrichment of calcrete. 

---